Sensitivity of the Endocochlear Potential Level to Cochlear Blood Flow during Hypoventilation

Abstract

To study the relationship between endocochlear DC potential (EP) and cochlear blood flow (CoBF) under hypoxic conditions, we recorded the EP and CoBF from the basal turn of the cochlea in 21 guinea pigs. Hypoventilation for 10 minutes was induced by reducing the respiratory rate and volume. During hypoventilation, the EP declined in most of the cases to an intermediate level of the positive range in a few minutes. At the midpoint of the 10-minute hypoventilation, angiotensin II (5 microg/kg or 1 mL/kg) was infused for 60 seconds to raise the systemic blood pressure. In this experimental manipulation of systemic blood pressure, the CoBF and EP generally rose transiently. We determined the sensitivity of the EP to a CoBF change (delta) by calculating the deltaEP/deltaCoBF. More specifically, we analyzed the relationship between the deltaEP/deltaCoBF and the EPi (EP level just before angiotensin II infusion). The deltaEP was equal to the maximum EP level after angiotensin II infusion minus the EPi. The deltaCoBF was equal to the maximum CoBF value after angiotensin II infusion minus the CoBF value just before infusion. The deltaEP/deltaCoBF increased most in the range near 70% of the EPi. That is, the deltaEP/deltaCoBF was greater and the EPi was lower in the range above 70% of the EPi. To elucidate this linear correlation in the range above 70% of the EPi, we must consider several factors. In the supplementary experiments for blood gas analysis using 11 guinea pigs, most of the data of the EPi in the range above 70% were found to be obtained under conditions of a PaO2 of more than 12 mm Hg. As to the sensitivity increase of the EP to the deltaCoBF above mentioned, we propose that among several factors in the stria vascularis during hypoxemia, the activation of glycolysis in aerobic metabolism may be involved. As another possible factor, we postulate the increase in the reactive rate of the enzymatic activities that are linked with EP production and respond to the elevated cyclic adenosine monophosphate activity induced by the sympathicotonic state due to hypercapnia.

Perilymph Osmolality Modulates Cochlear Function

“…Additionally, in the first study, the authors could not identify a change in cochlear blood flow with their methodology, and in the second cochlear blood flow was not assessed. However, others have reported that serum osmolality modulates cochlear blood flow and the EP, which can change cochlear function 27–29 . In contrast, we directly changed the perilymph osmolality and demonstrated that the EP did not change during our manipulations.…”

Functional expression of P2X4 receptor in capillary endothelial cells of the cochlear spiral ligament and its role in regulating the capillary diameter

“…However, the cellular mechanism underlying the ATP-induced regional blood flow increase has not been studied. The SL capillaries are exposed to perilymph and perform an important function of controlling the blood flow in the SV, which is critical to generate EP (46,31,63,62,37,32). In this study, we found that topical perilymphatic superfusion of ATP (1 mM) dilated a single segment of the SL capillaries by 11.5%, which is a substantial change for the SL capillary network considering the power relationship of diameter change to flow resistance for the capillaries.…”

“…The cochlear lateral wall expends energy to generate the endocochlear potential (EP), an important component of the driving force for the hair cell transduction current, which is essential for normal hearing. EP generation is highly dependent on the oxygen that is provided by blood flow through microcirculation in the cochlear lateral wall (57,14,46,31,63,62,37,32). The vulnerability of the EP to hypoxia and ischemia suggests that the blood flow in the cochlear lateral wall is dynamically and precisely regulated to meet the changing metabolic needs of the cochlear lateral wall.…”

Alterations in cochlear function during induced acute hyperinsulinemia in an animal model

“…The inner ear is not able to store much energy. 11 , 12 Paradoxically, it has a high metabolic activity, especially for maintaining endolymph ionic concentrations. 13 , 14 Mendelsohn and Roderique showed that induction of hypoglycemia in rodents reduces significantly endolymph potassium concentrations.…”